The frequency spectrum that is shared among radio communication devices is limited. Thus the ability of a transmitter to transmit as much information as possible in an allocated frequency spectrum or channel without interfering with other communication devices in adjacent channels is of great importance. To transmit as much information as possible in the allocated channel, digital communication systems typically modulate both the amplitude and phase of a radio frequency (RF) carrier. The amplitude modulation allows more information to be encoded on the carrier in a given channel than if only the phase was modulated. However, the amplitude modulation puts additional requirements on the transmitter that would not exist if only the phase of the RF carrier was modulated.
These additional requirements are due to the inherent nonlinear effects resulting from the amplification of an amplitude-modulated signal by an RF power amplifier. Due to the nonlinear characteristics of the RF power amplifier, signal distortion components that include an amplitude component and a phase component are added to the original signal. A number of prior art signal processing techniques have been developed to compensate for the nonlinear characteristics of RF power amplifiers. One such technique involves the use of a feed forward correction circuit in a feed forward amplifier. In general, feed forward amplifiers separate out distortion components generated by the RF power amplifier to create an error signal. The error signal is then amplified and added to the RF power amplifier's output with an amplitude, phase, and delay adjusted for maximum cancellation of the distortion components.
FIG. 1 is a block diagram of an exemplary feed forward amplifier 100 of the prior art. In feed forward amplifier 100, an input signal 102 having carrier components is routed to a radio frequency (RF) power amplifier 106 via an input signal coupler 104. RF power amplifier 106 amplifies the signal to produce an amplified signal 107. As mentioned above, RF power amplifier 106 introduces distortion components to the amplified signal, which distortion components are partially cancelled by an error signal output by a feed forward correction circuit.
The feed forward correction circuit includes a summation junction 116, a gain and phase adjuster 118, and an error amplifier 120. Summation junction 116 receives a portion of input signal 102 via input signal coupler 104 and further receives a portion of amplified signal 107 via a first output signal coupler 108 coupled to an output of RF power amplifier 106. Summation junction 116 subtracts the received portion of the amplified signal from the received portion of the input signal to produce an error signal 117. The subtraction results in a partial cancellation of the carrier components of the received portion of amplified signal by the carrier components of the received portion of the input signal. As a result, error signal 117 primarily contains the distortion components of the received portion of the amplified signal.
Summation junction 116 conveys error signal 117 to error amplifier 120 via a feed forward gain and phase adjuster 118. Gain and phase adjuster 118 adjusts a gain and/or phase of error signal 117 based on a control signal 129 in order to facilitate a cancellation of the distortion components of amplified signal 107 by an amplified and gain and/or phase adjusted version of error signal 117. Gain and phase adjuster 118 then conveys the gain and/or phase adjusted error signal to error amplifier 120. Error amplifier 120 amplifies the received error signal to produce an amplified error signal 121 and conveys the amplified error signal to an output signal combiner 112. Output signal combiner 112 combines amplified error signal 121 with amplified signal 107 to partially cancel the distortion components of the amplified signal and produce a distortion reduced output signal 113.
Control signal 129 is generated by a control circuit comprising a power detector 124, a correlator 126, and an integrator 128. A second output signal coupler 114 samples output signal 113 to produce an attenuated output signal 123. Second output signal coupler 114 conveys attenuated output signal 123 to detector 124. Detector 124 detects a power of attenuated output signal 123 to produce a detected signal 125 that the detector conveys to correlator 126. Correlator 126 correlates detected signal 125 to a known reference signal to produce a correlation signal 127 that the correlator conveys to integrator 128. Integrator 128 integrates correlation signal 127 to produce control signal 129. Control signal 129 is then used by gain and phase adjuster 118 to adjust the gain and/or phase of error signal 117, thereby adjusting the error signal based on distortion components detected in output signal 113.
In a code division multiple access (CDMA) communication system, the bandwidth of integrator 128 must be very narrow in order to filter an audio pilot tone from a noise-like CDMA signal. The narrow bandwidth of the integrator limits the slew rate of correlator 126 such that the time that it takes control signal 129 to converge from a “rail,” or initial, position to an optimal (near midrange) position when amplifier 100 is turned on or keyed up may be as long as 30 seconds. Furthermore, the control circuit of feed forward amplifier 100 is not capable of pausing, that is, of holding a control signal at a determined level, while the feed forward amplifier discontinues operation in the event that a performance of the feed forward amplifier is outside of an acceptable range.
FIG. 2 is a block diagram of another exemplary feed forward amplifier 200 of the prior art. Feed forward amplifier 200 operates in a manner similar to the operation of feed forward amplifier 100 except that feed forward amplifier 200 digitally generates a control signal 207. That is, in feed forward amplifier 200, an attenuated output signal 123 is conveyed to a detector 202 that detects a power of attenuated output signal 123 and produces a first digital signal 203 corresponding to the detected signal. Detector 202 conveys detected signal 203 to a microprocessor 204 that correlates the detected signal to a known reference value to produce a second digital signal 205 corresponding to the correlation. Microprocessor 204 then conveys digital signal 205 to a digital-to-analog converter (D/A) 206 that converts digital signal 205 to an analog signal to produce control signal 207. Control signal 207 is then used by gain and phase adjuster 118 to adjust the gain and/or phase of error signal 117.
Unlike feed forward amplifier 100, feed forward amplifier 200 is capable of pausing, that is, of holding a control signal at a determined level, while the feed forward amplifier discontinues operation in the event that a performance of the feed forward amplifier is outside of an acceptable range. Furthermore, unlike feed forward amplifier 100, feed forward amplifier 200 provides a control signal 207 that can start at any point, eliminating the need to converge from a “rail” position. However, the adjustments provided by feed forward amplifier 200 are limited to discrete step sizes and are further limited by a time delay between adjustments, unlike the continuous and nearly instantaneous adjustments of feed forward amplifier 100. Furthermore, D/A 206 limits the error correction provided by the feed forward correction circuit and control circuit of feed forward amplifier 200. In addition, once converged, the control circuit must perturb the system up or down (+/−) one step to confirm the level at which feed forward amplifier 200 has converged.
Therefore a need exists for a method and an apparatus in feed forward amplifier that provides for pausing control and continuous adjustment of a control signal, provides for the control signal to start at any point, provides for rapid convergence, and that does not perturb the system up or down one step to confirm the convergence of the system.